[docker-py] Add some return types to DockerClient method

[roberfi]

I've used info from stubs/docker/docker/api/daemon.pyi to annotate the following methods: df, events, info, login, ping and version.

Besides, to add the Generic type for events method, I've checked the behavior of that method in
https://github.com/docker/docker-py/blob/62bd22d9b7581157e118e5bc69f391f3331b3c6d/docker/api/daemon.py#L24, where _stream_helper takes decode argument to return text or json() from response.

On the other hand, the method close comes from requests.Session and returns None :
https://github.com/psf/requests/blob/70298332899f25826e35e42f8d83425124f755a5/src/requests/sessions.py#L794

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[srittau]

Thanks!